“Materials and methods employed to
validate a CT scanner’s approximation
of effective atomic number”
Robert Loader
Matthew Dixon
Lauren Taylor
Clinical & Radiation Physics
Directorate of Healthcare Science & Technology
Plymouth Hospitals NHS Trust
Dual Energy imaging
The Claims….
•
•
•
•
•
•
I/H2O/Ca/Other quantification
Virtual non contrast imaging
KeV data sets (contrast optimisation)
Beam hardening reduction
ROI defined spectral Hounsfield unit curves
Calcium subtraction
• Measurement of effective atomic
number (Z)
The pictures
Percentage of pixels vs Effective Z
HU vs Energy
The use? – Local work
12
1
2
3
4
5
6
7
8
10
Z (ROI)
14
16
The use? – Local work
1
2
3
4
5
Type of Stone
6
Cystine
Uric_acid
Struvite_stone
Calcium_Oxalate
Calcium_Phosphate
Predominant_Calcium_Oxalate
Predominant_Calcium_Phosphate
Brushite
7
8
Local work
• Only Uric acid separation possible
• Effective Z appears to increase separation from HU at 80 and
140KV
• To bear in mind, the real difference in effective atomic number
among the different non uric acid stone is small (especially for mixed
stones), so can we accurately predict atomic number and their
differences?
• Further large sample analysis needed.
The validation
1. How does GSI measure/calculate ROI
effective Z?
2. Scan known materials and get the
scanner to tell us the effective atomic
number!
3. Compare the difference
The Problem…
• Need to test over a suitable range of Z’s
• Difficult to get hold of “pure” elements
Help from the Chemists!
How does the Scanner calculate
the effective atomic number?
“The effective atomic number is defined in terms of the attenuation
“signature” for a material. That is, if the material that we scan behaves
like a periodic element when an atomic number X, then we say that the
effective atomic number of that material is X”
How does the Scanner calculate
the effective atomic number?
• Scanner uses the ratio of total attenuation at
70KeV and 120KeV.
• It matches this identified ratio to a point on the
curve of known elements of known atomic
number.
• This is then used as a calibration curve to
calculate the effective atomic number of
unknown compounds, elements and mixtures.
Scanner tells us Z, use NIST to get the attenuation ratio of
70KeV/120KeV
Example (for Iodine mixtures)
Example (for Iodine mixtures)
Example (for Iodine mixtures)
The materials
•
•
•
•
•
•
•
Water
Carbon (graphite)
Air
Aluminium
Chalk
Iodine concentrations
Our Chemistry set ☺
Iodine concentration in
16cm Perspex phantom
Elements and mixtures
Materials:
1. Water
2. Iodine
3. Carbon
4. Chalk
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
85
Effective Atomic Number (Z) ROI analysis.
80
75
Hf= 65
W = 70
70
Re = 72
65
Os = 74
60
Pt = 78
55
50
45
K-edge of element
disrupting attenuation
ratio
Au = 81
Pb = 88
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
85
Effective Atomic Number (Z) ROI analysis.
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Water (H2O)
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
85
Effective Atomic Number (Z) ROI analysis.
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Carbon (graphite bush)
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
85
Effective Atomic Number (Z) ROI analysis.
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Chalk (CaCO3)
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
85
Effective Atomic Number (Z) ROI analysis.
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Aluminium
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
85
Effective Atomic Number (Z) ROI analysis.
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Air (78N21O1AR)
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
Effective Atomic Number (Z) ROI analysis.
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Water (H2O)
Carbon (graphite bush)
Chalk (CaCO3)
Aluminium
Air (78N21O1AR)
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
Effective Atomic Number (Z) ROI analysis.
20
15
10
5
Air
0
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Water (H2O)
Carbon (graphite bush)
Chalk (CaCO3)
Aluminium
Air (78N21O1AR)
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
Effective Atomic Number (Z) ROI analysis.
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Chemistry mixtures (Various)
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
Effective Atomic Number (Z) ROI analysis.
85
80
75
70
65
60
55
50
45
40
35
30
25
20
? Saturation
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Ioversol (opiray) C18H24I3N3O4
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
85
Effective Atomic Number (Z) ROI
analysis.
80
75
y = 5.4773x + 6.9908
70
R = 0.8662
2
65
60
y = 8.3386x + 0.8174
2
R = 0.9983
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
linear NIST
Linear (linear NIST)
Ioversol (opiray) C18H24I3N3O4
Linear data
Linear (Linear data)
Effective Atomic Number (Z) ROI analysis.
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
dz~4.5
dz~2.2
? Scanner measurement
? Our error (% Ioversol by
weight)
3.2
Ioversol (opiray) C18H24I3N3O4
3.1
NIST data Elements
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
Attenuation Coefficient Ratio (70keV/120keV) NIST
Effective atomic number (Z) measures for a range of elements, compounds
and mixtures utilising an HD750 discovery series CT scanner (GE
Healthcare)
90
85
Effective Atomic Number (Z) ROI analysis.
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0
5
4.9
4.8
4.7
4.6
4.5
4.4
4.3
4.2
4.1
4
3.9
3.8
3.7
3.6
3.5
3.4
3.3
3.2
3.1
3
2.9
2.8
2.7
2.6
2.5
2.4
2.3
2.2
2.1
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Attenuation Coefficient Ratio (70keV/120keV) NIST
NIST data Elements
Chemistry mixtures (Various)
Water (H2O)
Carbon (graphite bush)
Chalk (CaCO3)
Aluminium
Air (78N21O1AR)
Ioversol (opiray) C18H24I3N3O4
Discussion
• With the exception of air, for the elements
scanned, the CT scanner’s measure of atomic
number was in reasonable agreement with
known elements.
• Chemistry mixtures and iodine concentrations
broadly followed the expected “signature” curve.
• Further work is required to assess a possible
saturation in scanner measured Z for high iodine
concentrations.
• Further work is required to calculate total
measurement uncertainty before firm
conclusions can be made.
A Question…
• Based on these early measurements,
would I expect the scanner to accurately
measure a small change in Z within a
small ROI over a kidney stone?
THE END